When you have to run resistive heaters the electrical usage skyrockets and makes a heat pump system vastly more expensive to operate.

More expensive than a heat pump without a backup/additional electrical resistance heater. Not more expensive than an electrical resistance heater on its own.

That's what's so strange about this, in most cases a heat pump would be replacing a non-reversible AC and an electrical resistance heater or gas-fueled furnace. And in nearly every case, even with the need for a backup/additional resistance heater, you're still saving money. And it's not like any of this is new technology, it just took forever for it to become popular in the US.